feat: Add Stock Items UI and update Supplier offerings

- Add Stock Items index page listing all stock items
- Add Stock Items edit page with supplier offerings management
- Update Suppliers edit page to manage offerings (select from stock
  items instead of material+length)
- Add Stock Items navigation link to sidebar

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CutList.Web/Components/Layout/NavMenu.razor

@@ -23,6 +23,11 @@
                 <span class="bi bi-box-nav-menu" aria-hidden="true"></span> Materials
             </NavLink>
         </div>
+        <div class="nav-item px-3">
+            <NavLink class="nav-link" href="stock">
+                <span class="bi bi-boxes-nav-menu" aria-hidden="true"></span> Stock Items
+            </NavLink>
+        </div>
         <div class="nav-item px-3">
             <NavLink class="nav-link" href="suppliers">
                 <span class="bi bi-building-nav-menu" aria-hidden="true"></span> Suppliers